This invention relates to low inductance circuit apparatus, and particularly to circuit apparatus having connectors for connecting wiring boards to a backpanel. More particularly, the invention relates to a rigid/flex wiring board with a connector connecting the flex portion to a backpanel.
Prior circuit connection arrangements formed inductive loops in the signal paths in circuits between separate printed wiring boards connected by backpanels. The loop areas can be approximated by the cross-sectional area formed by the physical length and separation of a signal path from its ground return path. These loops form inductive reactances which, at high frequencies, deteriorate signals in the signal paths and induce undesirable voltages in the ground planes. The inductance is proportional to the area of the loop made by the current flow; a larger loop forming a greater inductance, hence more adversely deteriorating signals in the signal paths at a given frequency. Induced signals in the ground plane create undesirable ground shift.
In digital electronics employing pulse signals with "square" profiles, it is important to maintain sharp rise and fall times to the pulse signal. However, inductive loops tend to lengthen the rise and fall times, thereby deteriorating the quality of the pulse signal. To minimize deterioration of signals in the signal paths of printed wiring boards and backpanel interconnections, it is desirable to maintain the inductive loop as small as possible.
Another problem with prior flex circuits is that the mechanical connection between the flex circuit and the rigid wiring boards resulted in unacceptable impedances in the signal paths, causing deterioration of the signals in the signal paths.
The quality and symmetry of the ground pin assignment across the connector interface is a major factor in determining effective connector inductance. Marginal grounding will cause ground shift, especially at unswitched outputs, resulting in deterioration of signals and erroneous data transfer. Accordingly, it is important to consider the ground pin assignment to minimize ground shift.
Young U.S. Pat. No. 4,755,147, issued Jul. 5, 1988 and assigned to the same assignee as the present invention, describes a flexible wiring circuit (flex circuit) having a circuit trace plane and a ground plane. The circuit trace plane includes a plurality of circuit traces, some of which extend between logic trace holes in the circuit trace plane and leads connected to one or the other of two rigid wiring boards. Others of the circuit traces connect between leads connected to each rigid wiring board to provide signal paths between the two wiring boards. The logic trace holes are arranged to connect to individual pins of a pin connector for input/output to a backpanel. The ground plane includes a plurality of ground plane holes for connection to other individual pins of the pin connector. The ground plane is separately connected to individual leads connected to each rigid wiring board. The leads connecting the ground plane and circuit traces to the rigid wiring boards are solder connected to individual pads on the respective wiring board. The backpanel provides interconnection to the pins of the connector in a manner dictated by the circuit traces and configurations of the backpanel.
The flex circuit and connector described in the Young patent provided a substantial advance in flex circuit connectors. The pattern of ground connection pins between the pin connector and ground plane in the Young patent was such as to reduce impedance mismatch between circuits on the rigid wiring boards and circuits on the backpanel, and to reduce inductance between the ground plane of the flex circuit and the ground of the backpanel thereby reducing local ground potential shifts when operating circuits.
Although the Young patent optimized the loop size in the flexible circuit interconnection between each wiring board and the backpanel, the Young approach did not fully optimize loop size through the connection between wiring boards, nor to the wiring boards themselves. Moreover, the Young arrangement continued to exhibit problems with high frequency inductance and ground shifts. The solder connection of the leads to the pads on the rigid wiring boards created impedance mismatch, particularly in the ground plane connection. The use of the leads to connect the ground plane to pads on the rigid circuit boards limited the space available for signal trace connections to the wiring boards, thereby limiting the input/output connections to the backpanel and the number of crossunder connectors between boards. While this latter drawback could be reduced by simply dedicating more leads to signal traces and less to ground connections, such a solution diminished the integrity of the ground connections. Further, the use of the connection holes in the individual ground and signal trace planes of the flex circuit, together with solder connections to the pins of the pin connector, effectively limited the backpanel connection through the pin connector to only two planes on the flex circuit: the ground plane and the single signal trace plane. Hence, the Young patent could not accommodate a power plane, nor more than one signal trace plane.